In conventional telecommunications, discrete circuits perform discrete duties and are non-overlapping. Power amplification is performed solely by a power amplifier, and switching is performed solely by a mode switch.
FIG. 1 illustrates a high level conventional combination of a power amplifier and a mode switch in a 3G (third generation) radiofrequency chain of a cellular handset. FIG. 1 conveys the main elements of the conventional implementation and will serve to differentiate the concepts of the present disclosure, which will be demonstrated later.
In FIG. 1 (from left to right), power amplifiers (PA1 and PA2) feed into mode switches (SW1 and SW2 respectively); the mode switches feed into duplexers (D1, D2, D8, or D5); the duplexers feed into a selector switch (SW3); and the selector switch selectively feeds into an antenna ANT.
The top half of FIG. 1 is associated with high band transmissions. Specifically, power amplifier PA1 amplifies the power of a high band (HB) wide band code division multiple access (WCDMA) signal VHBIN, and outputs VHBOUT.
As shown in the top half of FIG. 1, VHBOUT is transmitted (without amplification) through mode switch SW1, duplexer D1 (for transmissions in band 1), selector switch SW3, and finally to antenna ANT. Duplexer D1 may perform some filtering. Duplexer D2 (for transmissions in band 2) is not currently selected.
Similarly, as shown in the bottom half of FIG. 1, VLBOUT is transmitted (without amplification) through mode switch SW2, duplexer D8 (for band 8), and is received by selector switch SW3. Duplexer D8 may perform some filtering. Duplexer D5 (for band 5) is not currently selected.
If selector switch SW3 selected the duplexer D8 (this selection is not shown), then VLBOUT would be transmitted in band 8 (without amplification) to the antenna ANT.
In FIG. 1, there are two WCDMA Power Amplifiers, one for High Band (PA1) and one for Low Band (PA2). Low Band typically refers to the cellular frequency bands in the 0.7-1.0 GHz range, and High Band generally implies operation in the 1700-2100 GHz range, although many different frequency bands are being proposed in next generation cellular systems. Each one of these PA outputs (VHBOUT and VLBOUT respectively) is then routed to a Mode Switch (SW1 and SW2 respectively), which is then routed to the appropriate TX/RX duplexer. For each WCDMA band supported, there will typically be a throw or position in the mode switch so that its TX signal can be routed to the appropriate duplexer and directed to the handset antenna. Although FIG. 1 shows only 4 WCDMA bands (two LOW Bands #5 and 8# and two High Bands #1 and #2), modern smart handsets with 10 or more frequency bands have already been launched or are in the process of being developed.
If one were to focus on the Power Amplifier (PA1 and PA2) and Mode Switch (SW1 and SW2) components, at the transistor level, one would typically encounter the arrangement shown below in FIG. 2. The conventional implementation of the 3G PA followed by a mode switch depicted in FIG. 2 shows many of the traditional concepts known to those skilled in the RF cellular design area. Several results of this conventional implementation are noted below since they will be later contrasted with the concepts of the present disclosure.
FIG. 2 illustrates a transistor level implementation of a conventional combination of a power amplifier PA1 (matched to 50 ohms) and a mode switch SW1 in a 3G (third generation) radiofrequency chain of a cellular handset.
In FIG. 2, power amplifier PA1 receives signal VHBIN and outputs fully amplified signal VHBOUT. Specifically, power amplifier PA1 includes input matching network N4, transistor T2, interstage matching network N6, transistor T4, and output matching network N8 in series. DC controller and biasing network N2 receives control lines CL1 and receives voltage VBATTERY. The input impedance Z2 to power amplifier PA1 is matched at 50 ohms. The output impedance Z6 for power amplifier PA1 is 50 ohms. The output impedance of the second transistor T4 is Z4, and is typically 2 to 4 ohms for a large second transistor T4.
Power amplifier PA1 may amplify from 30 to 32 dB, and output matching network N8 may lose approximately 20% of the power required to operate power amplifier PA1. Thus, this power amplifier PA1 is very lossy.
Mode switch SW1 acts as a simple switch, receives fully amplified signal VHBOUT, and transmits VHBOUT (without additional amplification) towards either duplexer D1 (as signal TX_B1 for band1) or towards duplexer D2 (as signal TX_B2 for band2), depending upon which mode (band1 or band2) has been selected (D1 and D2 are shown in FIG. 1).
Mode switch SW1 includes a first series portion Series_1, a first shunt portion Shunt_1, a second series portion Series_2, a second shunt portion Shunt_2, and a DC bias controller and biasing network N10.
The first series portion receives VHBOUT and outputs TX_B1. The first series portion includes a series of transistors (T10, T12, and T14) with gates connected to gate voltage VGseries_Port1, and with bodies connected to a body voltage VBseries_1.
The first shunt portion Shunt_1 includes a series of transistors (T20, T22, and T24) with gates connected to VGshunt_Port1, and with bodies connected to VBshunt_Port1.
The second series portion receives VHBOUT and outputs TX_B2. The second series portion includes a series of transistors (T30, T32, and T34) with gates connected to VGseries_Port2 and with bodies connected to VBseries_Port2.
Second shunt portion Shunt_2 includes a series of transistors (T40, T42, and T44) with gates connected to VGshunt_Port2 and with bodies connected to VBshunt_Port2.
Mode switch SW1 is a two mode switch, configured to switch VHBOUT to a first band (band1) or to a second band (band2).
If the first band (band1) is selected, then VHBOUT is switched to first band (band1) at TX_B1 with low impedance (but with no amplification). The first series portion series_1 is ON, the first shunt portion shunt_1 is OFF (isolating TX_B1 from ground), the second series portion series_2 is OFF (isolating TX_B2 from VHBOUT), and the second shunt portion shunt_2 is ON.
Specifically, the first series portion series_1 is ON (closed circuited, with gate voltage VGseries_Port set to +2.5V, with body voltage VBseries_Port1 set to 0V, and with an impedance of approximately 0.5V), and the first shunt portion shunt_1 is OFF (open circuited, with a gate voltage VGshunt_Port1 of −2.5V, with a body voltage VBshunt_Port1 set to −2.5V, and with a high impedance). Further, the second series portion series_2 is OFF (open circuited, with gate voltage VGseries_Port2 set to −2.5V, with body voltage VBseries_Port2 set to −2.5V, and with high impedance). The second shunt portion shunt_2 is ON (short circuited, with gate voltage VGshunt_Port2 set to +2.5V, with body voltage VBshunt_Port2 set to 0V, and with low impedance).
If the second band (band2) is selected by mode switch SW1, then VHBOUT is switched to a second band (band2) at TX_B2. Further, the states of the four portions are the opposite of what they were when the first band was selected. Thus, the first series portion series_1 is OFF, the first shunt portion shunt_1 is ON, the second series portion series_2 is ON, and the second shunt portion shunt_2 is OFF.
In FIG. 2, the power amplifier PA1 is implemented with a dual stage HBT (heterojunction bipolar transistor) device technology (see NPN bipolar transistors T2 and T4). Traditionally GaAs or silicon HBTs devices are employed in the industry, although GaAs and silicon FETs can also be used for this function. The final stage of the HBT section shown is typically built with a very large RF power device T4. Typically for a 3G WCDMA application, emitter areas of 4000-8000 μm2 are used for T4, which cause the output impedance Z4 of the HBT transistor T4 device to be very low, typically in the range of 2-4 Ohms.
In the conventional implementation of FIG. 2, Output Matching Network N8 of power amplifier PA1 must convert the output impedance of the HBT T4 as discussed above to a value close to 50 Ohms in the band of operation of power amplifier PA1. Because this impedance transformation is very large (typically 2 ohms transformed to 50 ohms) and needs to occur over a large range of frequencies where the power amplifier PA1 is supposed to operate, the Output Matching Network N8 ends up incurring unnecessary loss and complexity, especially when one considers that such match must also support harmonic terminations and other networks needed for amplifier stability. It would indeed be much more desirable to have the output impedance of the final stage of the device T4 be much closer to 50 Ohms for ease of matching and minimization of loss and stability.
Mode switch SW1 is shown in FIG. 2 implemented with SOI CMOS technology, although other FET technologies such as GaAs PHEMTs, bulk CMOS, etc., could be used.
In FIG. 2, the input terminal of the mode switch SW1 must be able to reliably withstand the largest output power generated by the power amplifier PA1. Those skilled in the design of RF power switches recognize that multiple SOI FETs connected in series with a stacking number are needed to provide reliable switch operation for a given power level. For cellular handset power levels, one typically stacks 12-14 NFET devices of 0.25 um in series, as an example.
In FIG. 2, shunt portions Shunt_1 and Shunt_2 are placed at the output terminals (TX_B1 and TX_B2) of mode switch SW1 to provide the required port-to-port isolation. The shunt portions must also be stacked as well so as to provide the required isolation.
In FIG. 2, the gate and body voltages of the different mode switch portions are controlled by a controller network N10 (internal or external) so as to provide the required voltage levels according to the mode in which the switch needs to operate.
In FIG. 2, the large number of FETs required for the handling of the RF power levels creates high loss due to the composite ON resistance of the branches that are turned ON, due to the parasitic effects of the other FETs which are turned OFF, and due to the impedance mismatch that the mode switch SW1 inserts between the power amplifier PA1 and the ports TX_B1 and TX_B2.
In summary, the conventional implementation of a power amplifier PA1 and a mode switch SW1 is inefficient because the series portions in the mode switch only serve as switches, and do not amplify. Further, the power amplifier PA1 must be very large and must provide all of the amplification. Also, every transistor in each series portion must be sized to handle the maximum power from power amplifier PA1.
The usage of some of the elements of a stacked FET amplifier and integration in a single or a multi branch switch are particularly beneficial, and are described in further detail below. Several of the benefits of this architecture will be described below and contrasted with the conventional approaches in typical RF amplifier/switch arrangements used in the RF applications such as cellular handsets or Wi-Fi routers.